SO leaves and realizes they made mistake

Hi ladies, have you ever gone through this ?

So leaves but realize they made mistake and wants to come back? But you knew they were making a mistake leaving. Did you let them come back ? And how did you move forward from this ?

My SO wants to leave. I know he is making a mistake but I also don’t want to be one of those women who tell him, I’ll be here when he wants to come back, because I think it’s very unhealthy letting someone know they can come and go as they please.

I can honestly move forward but I do love him. I just want to know how to go about this the right way if there is even a way to do that. I am thinking the future and I just don’t think I can be able to forgive him wanting back in after he leaves. But I also know there are lots of people who’ve gone through this and that maybe that’s what he needs to see how shitty it is out there to change etc idk.

I’ll really appreciate your thoughts